The invention relates to a laser amplifier arrangement, in particular a miniaturized laser amplifier arrangement.
The invention relates in particular to a compact fiber-coupled solid-state laser oscillator/laser amplifier for generating laser radiation with a high beam quality and a high power.
For many applications, it is desirable or even necessary to have miniaturized lasers that can generate pulsed laser radiation with pulse widths of a few nanoseconds and pulse energies in the range of several mJ. Examples of applications are long-range laser measurement systems, lasers for precision material processing or for exciting optically nonlinear processes. Diode-pumped solid-state lasers are particularly suitable for this. For further details in this regard, reference is made to P. Peuser, N. P. Schmitt: “Diodengepumpte Festkörperlaser” [Diode-pumped solid-state lasers], Springer Verlag, 1995. The necessary pulse powers typically lie in the range of from about 100 kW to more than one MW.
It is possible to produce particularly compact or even miniaturized pulse lasers using passive Q-switching but at present reliable operation with a high beam quality and amplitude stability is possible only with maximum pulse energies of a few mJ. Corresponding pulse lasers are described in P. Peuser, W. Platz, P. Zeller, T. Brand, B. Köhler, M. Haag; Opt. Lett. 31 (2006) 1991. In order to achieve power scaling, one or more amplifiers with a multiple beam path (multipass) may be connected downstream, so that particularly large pulse energies are achieved. In this case, however, the possibilities of substantial miniaturization are reduced.
When the so-called longitudinal pump geometry can be applied for exciting the active material, optimal conditions can be provided for achieving high efficiency and good compactness. Further details in this regard may be found in the aforementioned literature reference P. Peuser, N. P. Schmitt: Diodengepumpte Festkörperlaser, Springer Verlag 1995.
For practically suitable laser systems, it is particularly advantageous when the coupling to the supply and control electronics can be established over a several meter long fiber connection.
Recently developed pulsed fiber lasers or fiber amplifier arrangements are distinguished by a very compact structure and a high beam quality, but the available pulse powers usually lie below 100 kW, which is no longer sufficient for many applications.
In this case, various fundamental physical processes limit the pulse energies to the range of typically about 1 mJ. These processes are primarily ASE (so-called amplified spontaneous emission), stimulated Brillouin scattering, stimulated Raman scattering and self-focusing. In this context, reference is made to F. D. Teodoro et al., Opt. Lett. 27 (2002) 518 and R. L. Farrow et al., Opt. Lett. 31 (2006) 3423 for further details. Owing to the small fiber cross section, extremely high intensities occur in ns pulsed operation, so that when the pump power is scaled the fiber is destroyed. In order to generate a few mJ, the fiber cross section must be increased to such an extent that the beam quality would be significantly reduced.
U.S. Pat. No. 6,553,052 B1 discloses a laser amplifier arrangement comprising a laser oscillator and a laser amplifier, wherein the laser oscillator and the laser amplifier are pumped by a common laser diode. The amplified laser beam is coupled laterally out of the laser amplifier arrangement. The pump beam of the laser diode first enters the laser amplifier, in order to pump it. Remaining pump radiation, which passes through the laser amplifier (transmission) is used for pumping the laser oscillator in order to generate the laser radiation to be amplified. To this end, the laser amplifier is arranged between the laser oscillator and the pump source. The remaining part of the pump radiation, passing through the laser amplifier, is introduced into the laser oscillator at a front end, through which the laser radiation to be amplified also emerges from the laser oscillator, before then being introduced into the laser amplifier. The amplified laser radiation, emerging from the end of the laser amplifier directed toward the pump source, must then be coupled out laterally since the pump source stands in the way in the direction of the longitudinal axis. In order to couple the residual pump radiation into the laser oscillator, beam optics with stops are provided, which comprise lenses having central opening for unimpeded passage of the laser oscillator radiation travelling back.
Although this configuration makes it possible for a large part of the pump radiation to be available for the laser amplification, it places significant limits on miniaturization.
U.S. Pat. No. 6,373,864 B1 discloses a laser amplifier arrangement suitable as a fully passive laser system for generating and amplifying short pulses with a high repetition rate. To this end, a microchip laser is provided as a laser oscillator, which is optimized for generating short laser pulses with a high repetition rate. To this end, its resonator length is selected to be very short in order to be able to generate short pulses. Furthermore, a first pump source for the laser oscillator is optimized with a view to a high repetition rate. To this end, the first pump source is provided with a particularly bright pump light source. The pump radiation generated by the first pump source is coupled by first pump beam optics, or in an alternative embodiment by a first light guide fiber, into the laser oscillator at a first longitudinal end. The laser radiation to be amplified then emerges at the opposite longitudinal end of the laser oscillator, and is introduced by focusing optics into a laser amplifier crystal, arranged offset with respect to the longitudinal axis of the laser oscillator, at its first longitudinal end. At the opposite second longitudinal end, the laser amplifier crystal is provided with a reflector for the laser radiation to be amplified, so that the amplified laser beam is in turn directed back through the first longitudinal end, emerges again offset with respect to the incoming laser radiation and is output by the focusing optics. The laser amplifier crystal is pumped by a second pump source, which is optimized with respect to the light polarization and the radiation for amplification, in order to achieve a particularly high gain. This second pump source comprises a second pump beam source and its own second pump beam optics, by which the second pump radiation thereby generated is introduced, in the opposite direction and offset in terms of the longitudinal axes with respect to the first pump radiation, into the laser oscillator/amplifier configuration formed by the laser oscillator and the laser amplification crystal. Specifically, the introduction takes place at the second longitudinal end of the laser amplification crystal, which is provided with the reflective coating for reflecting the laser radiation to be amplified.
This reflective coating is intended to be used to guide the laser beam through the laser amplification crystal two times, so as to increase the gain. Like the first pump beam guide device, the second pump beam guide device may comprise a light guide fiber. Yet even in the case of a light guide fiber, this second pump radiation is introduced through the second longitudinal end of the laser amplifier crystal, while the first pump radiation is introduced into the laser amplifier configuration at the opposite first longitudinal end of the laser oscillator, and furthermore with an offset.
Although the laser amplifier arrangement according to this prior art is optimized with a view to amplifying pulses which are as short as possible with a repetition rate which is as high as possible, it is entirely unsuitable for miniaturization.
U.S. Pat. No. 6,512,630 B1 describes a miniaturized configuration in which a so-called passively Q-switched microlaser, or in general miniaturized laser, is coupled to an amplifier. All of the pump radiation is coupled into the microlaser and thereby partially absorbed in the laser crystal. The transmitted residual pump radiation, not absorbed in the laser oscillator, is focused together with the laser beam generated by the microlaser into the amplifier crystal by means of a lens. The laser beam emerging from the oscillator is then amplified in the amplifier crystal. In this prior art, all of the pump radiation is provided by a single diode laser beam source.
However, such a configuration is not suitable for efficient scaling of the power, or pulse energy, as will be explained below. In this context, distinction may fundamentally be made between two different pump types: quasi continuous-wave or pumped excitation and continuous-wave excitation.
In the case of quasi continuous-wave excitation, a single pulse that contains the maximum possible energy may be generated, or several pulses of smaller energy may be generated during a pump cycle.
In the case of pulsed excitation, the following applies: when there is an increase in the pump power, the laser pulse is generated earlier by the oscillator, in relation to the start of a pump radiation pulse of constant length. Owing to this, the energy stored in the amplifier can subsequently no longer be used, and the overall gain can no longer be increased. Directly connected with this, there is also a reduction in the overall efficiency. Temporal adaptation of the oscillator pulse to the pump radiation duration is crucial for achieving maximum pump energy and a high efficiency.
When there is an increase in the pump power on the other hand, in general several pulses, which respectively contain a smaller amount of pulse energy, are generated during a pump cycle. The temporal spacing of the pulses is commensurately less when the pump power is higher. Accordingly, an increase in the total pump power, for achieving a higher gain, simultaneously leads to a change in the pulse rate.
Similar considerations apply for the case of continuous-wave excitation. When the overall pump power of the laser oscillator/amplifier arrangement is increased, the pulse rate is in this case increased together with a simultaneous reduction in the individual pulse energy of the pulses generated by the laser oscillator. Or, expressed another way, a change in the pump power or the gain causes a change in the pulse rate. Furthermore, the pulse width changes as well since the inversion density in the laser oscillator crystal also changes.
Exemplary embodiments of the present invention are directed to a high-power laser with controllable power and having high beam quality and a high efficiency, which can be miniaturized to a very great extent.
The laser amplifier arrangement according to the invention comprises an optical pump source and a laser oscillator/amplifier configuration, which can be pumped by pump radiation from the pump source. The laser oscillator/amplifier configuration is arranged axially. Owing to the axial arrangement along the laser axis, the dimensions of the configuration can be kept very small. The laser oscillator/amplifier configuration comprises a laser oscillator and a laser amplifier. The laser oscillator can be excited by pump radiation to emit a laser beam, which is guided to the laser amplifier and amplified there. To this end, the laser amplifier is likewise excited by pump radiation. In order to achieve a high degree of miniaturization, the laser oscillator and the laser amplifier are essentially arranged coaxially or collinearly with respect to a longitudinal axis of the laser oscillator/amplifier configuration.
The pump source comprises at least two beam sources. A first beam source generates first pump radiation for pumping the laser oscillator. At least one second beam source generates second pump radiation for the laser amplifier. Both pump radiations are introduced into the laser oscillator/amplifier configuration so that it is excited in the longitudinal direction.
To this end, a pump radiation guide device is provided, by means of which the two pump radiations can be introduced into the laser oscillator/amplifier configuration for longitudinal pumping essentially in the direction of the longitudinal axis.
Preferably, laser radiation from at least two or more fiber-coupled diode laser beam sources, which are operated independently of one another, is used in order to excite a compact, axially arranged laser oscillator/amplifier configuration (preferably a solid-state laser oscillator/amplifier configuration) in the longitudinal direction. It is in this case preferable to optically pump the laser oscillator and the amplifier independently of one another.
According to preferred configurations, the laser may be passively or actively Q-switched.
The effect achieved by this configuration is that a high-power laser with controllable power and having high beam quality and a high efficiency is achieved with a very high degree of miniaturization.
Advantageous uses of the laser amplifier arrangement and advantageous configurations thereof are:
a) robot-mounted laser systems,
b) miniaturized aircraft-mounted LIDAR systems,
c) laser transmitters for use in space or
d) pump lasers for optically nonlinear processes.
An extremely compact and optimally controllable laser configuration is preferably produced, with which high-power ns pulses can be generated. The configuration presented here furthermore makes it possible to use active Q-switching.
The pump configuration according to the invention is advantageously used in conjunction with optical deviation systems—for example with lenses and precision mechanical elements—which guide the second pump radiation past the laser oscillator to the laser amplifier. In this way, the laser oscillator and the amplifier can be optimized independently of one another and tuned to one another, so that laser radiation with a high beam quality, high-power laser pulses and a high efficiency can be generated.
Out of the overall pump radiation power of the pump source, the second pump radiation to be used for exciting the laser amplifier preferably has by far the largest part. This may, for example, be achieved by configuring the second radiation source as a high-power diode laser, while the first beam source may be a diode laser of lower power. In another advantageous configuration, the pump source for forming the second radiation source comprises a multiplicity of diode lasers, which together generate the second pump radiation.
The first and second pump radiations are preferably guided through an optical fiber line to the laser oscillator/amplifier configuration. For the purpose of greatest possible miniaturization, various configurations of this optical fiber line may be envisaged. A parallel arrangement of a first optical fiber for guiding the first pump radiation and a second optical fiber for guiding the second pump radiation may be selected, in such a way that the fibers lie closely next to one another. A configuration in which the first optical fiber is arranged centrally inside a second fiber arrangement is particularly preferred. In this way, the second pump radiation is introduced annularly around the first pump radiation. The first optical fiber may thus readily extend as far as the laser oscillator, in which case the second pump radiation shone in radially outside the first optical fiber may be guided, for example by means of an optical deviation device, radially outside around and past the laser oscillator in the longitudinal direction as far as the laser amplifier.
Such an arrangement may, for example, be produced by the second optical fiber annularly surrounding the first optical fiber.
Particularly in the event that the second beam source comprises a group of second diode lasers, the optical fiber line may also comprise a fiber bundle consisting of second optical fibers, the first optical fiber being arranged in the middle of this fiber bundle (it does not need to be exactly in the middle, although this is preferred).